A stretch is something that goes beyond a normal limit, or an exaggeration. It's commonly used in the phrase it's no stretch(of the imagination) to +verb. The expression by no stretch of the imagination is also common and means that something cannot be true, no matter how hard you try to imagine it.

E.g.

It's no stretch to say that this hotel is the best in Spain- No es ninguna exageración decir que este hotel es el mejor de España

It's no stretch of the imagination to think that he could be guilty- No es muy difícil imaginar que podría ser culpable

By no stretch of the imagination could you call him an writer- Bajo ningún concepto se le podría llamar escritor